We've had three quotes to repaint downstairs and one's nearly half the others, so what's the catch?

Usually there's a reason, and it's almost always in what the cheap quote leaves out rather than the painter being a bargain. Nine times out of ten the gap is prep. Filling, sanding back, caulking, a mist coat on any bare plaster, proper masking and dust sheets — that's the slow, unglamorous part, and it's what a clean finish actually rides on. A quote at half the others has often priced a single coat, or two coats straight over whatever's there, or a cheaper contract emulsion than the ones either side allowed for.
First thing I'd do is sit the three quotes side by side and check they're pricing the same job. Ask each decorator, in writing, how many coats, whether filling and prep are included, what paint and what finish, and whether ceilings and woodwork are in or extra. You'll often find the cheap one quietly isn't doing the ceilings, or is planning one coat on a colour change, which never covers properly.
The honest trade-off: a fast, cheaper painter can be perfectly fine on sound walls going back a similar colour. Push dark-to-light, or paint onto tired plaster, and skimped prep shows within weeks as flashing and filler grinning through.
For a whole downstairs you're realistically somewhere around £800 to £2,000-plus depending on the number of rooms and the state of the walls, with day rates sitting about £150 to £250. There's a fuller breakdown on our painting and decorating cost page. If one quote sits well below that band, it's pricing less work, not the same work cheaper.
If you want a couple more like-for-like quotes to judge it against, get free quotes and we'll match you with vetted local painters and decorators who'll spec the prep and the number of coats up front.